My house was built in the 19th century. It still has a few original window panes ("lites" or "lights" as they are still called on Ocracoke). The original lights can be identified because they are wavy. If you look out the window and move your head from side to side the trees and fences across the lane seem to shimmer and sway.
Glass is a peculiar material. Some call it a solid-like liquid; others call it a liquid-like solid. Although glass does "melt" slowly (very slowly), one mathematician estimates that it would take about ten million years for a window pane to get 5% thicker at the bottom.
In case you are interested in learning a bit more about glass, check out these two links:
Our latest Ocracoke Newsletter is the story of windmills on Ocracoke. You can read it here: http://www.villagecraftsmen.com/news012113.htm.